PEO coatings sparking new interest : Products Finishing Home » Articles » PEO coatings sparking new interest PEO coatings sparking new interest Article From: Products Finishing, Dr. James Curran, Professor from Cambridge University Posted on: 6/1/2011 Plasma electrolytic oxide surface treatment for aluminum, magnesium or titanium CLICK IMAGE TO ENLARGE (+) For nearly a century, materials manufacturers have known that metals exposed to demanding environments can be protected by a simple yet extremely effective process known as anodizing. By immersing metals such as aluminium, titanium and magnesium in a solution that conducts electricity and applying an electric current, the natural oxide on the metal surface thickens and creates a protective outer layer. Although anodized layers are typically limited to a few microns in thickness and are porous, this simple process can alter the surface appearance of a metal and, importantly, vastly improve its resistance to corrosion and wear. Today, anodized metals enjoy varied and far-reaching applications stretching from construction, through the automotive industry, to consumer products.

More recent modifications have improved this process still further: by using stronger electric fields in a process known as plasma electrolytic oxidation (PEO), a wider range of coatings can be produced, and performance can be enhanced. Coatings produced in this way on aluminium, for instance, incorporate a very hard, crystalline phase of aluminium oxide called corundum – known in its naturally occurring form as the minerals sapphire and ruby – that is second only in hardness to diamond. WHAT IS PEO? Plasma electrolytic oxide coatings are hard, dense, wear-resistant, and well-adhered oxide coatings for metals such as aluminium and magnesium. The process by which they are grown may also be referred to as micro-arc oxidation or spark discharge anodizing. Essentially, it involves the modification of a conventional anodically grown oxide film by the application of an electric field greater than the dielectric breakdown field for the oxide. Discharges occur, and the resulting plasma-chemical reactions contribute to the growth of the coating. Like anodizing, the PEO process requires immersion of the components in a bath, and uses electrical current to grow a uniform layer of oxide on the surface of the metal, but that’s where the similarity ends.

The electrolytes are non-toxic and are free from any heavy metals. They are typically just very dilute aqueous solutions, with just a few litres per gram of salts which can be disposed of at no environmental or commercial cost. The applied electrical currents are generally pulsed at high frequencies and at voltages of over 200V. These conditions are specially selected so as to give rise to millions of very short-live microscopic discharges – just like bolts of lightning – through the oxide layer. These expose the oxide to temperatures over twice as hot as the surface of the sun (Ref: Dunleavy). It is this intense local injection of energy which can be used to transform the oxide into high performance engineering ceramics such as crystalline alumina. Whereas conventional anodising can only produce amorphous aluminum oxide, the PEO process can produce a phase more than five times harder (1600-2000 HV0.1) , making the resulting coating far more wear resistant than a hard-anodised layer. The coating is therefore harder than any steels ‑ or even glass and sand ‑ so there are very few things which can scratch or erode it, even on magnesium.

There are many examples of PEO coating enabling the replacement of steel parts with aluminum parts, where the lighter replacement’s harder surface means it can even significantly out-last the original. One such example is sprockets for off-road bikes, which last three-to-four times as long as the heavier steel parts which they replace. Because the process involves melting and re-solidification of the ceramic, it has a very different structure from a conventional anodized layer. The layer doesn’t show the usual columnar pores and opening cracks on sharp corners; instead it gives continuous cover. This is particularly important in the corrosion protection of complex geometries such as threads, or the wear protection of deliberately rough or textured surfaces. Examples of such applications include aluminum winch drums for yacht racing, and a specially formulated black coating for Mavic’s high performance aluminum racing cycle rims. The absence of columnar pores or cracks in the coating structure also has benefits in terms of fatigue performance as PEO coatings usually result in minimal fatigue debit, and certainly less than any conventional anodising process.

This has been demonstrated across a wide range of aluminum, magnesium and titanium alloys, and even on more exotic alloys such as the AMC640xa metal matrix composite selected by the European Space Agency for large structural members on the International Space Station. Indeed, the ability of the PEO process to coat a wide range of alloys is itself an advantage: conventional anodising processes are less effective on high copper alloys such as 2XXX or 7XXX series aluminum, due to copper precipitates in the substrate inhibiting the growth of a protective oxide film but under PEO processing, there is no such limitation. CHROME FREE PROTECTION OF MAGNESIUM For years, chromate conversion coating has been the standard pre-treatment for paint in the corrosion protection of magnesium alloys, but environmental regulation is increasingly discouraging its use. PEO provides a non-toxic alternative as extensive testing has been undertaken to compare the relative performance of this and other candidate systems for chromate replacement. One such example is Ford Motor Co. research (reference: Blanchard) which concluded that PEO could even out-perform the chromates. As such, the PEO-based systems meet or exceed the AMS-2466 specification.

Although more costly than mere chemical passivation, some PEO processes are used in the automotive industry as a chromate replacement wherever corrosion-resistance standards are strong enough to demand it. This includes motorsport where it is the most widely used magnesium coating in Formula 1, but also several mainstream automotive applications such as the Corvette roof, processed by IHC in Detroit. In aerospace, PEO processes are qualified for use as a pre-treatment for resins for various helicopter gearboxes, where there is a need to meet or surpass AMS-2466 in terms of corrosion protection, with minimal fatigue debit. Some PEO processes for magnesium can harden the surface to over 900 HV0.1, and thus offer the additional benefit of enhanced scratch and wear protection. TITANIUM COATINGS Another recent aerospace approval is the use of certain PEO process for the refurbishment of civil airliner landing gear bearing carriers, made from Ti6Al4V. The refurbished parts offer improved wear performance over the original system (ref: Martini), and improved anti-galling protection.

The use of PEO on titanium, however, is predominantly as a more functional surface in such areas as dental implants ‑ where it presents significantly enhanced tissue adhesion ‑ and as a photocatalyst in water purification, and in photoelectrochemical cells. These are just a couple of the examples of PEO’s increasingly diverse engineering solutions, which also include thermal management, high or low temperature electrical insulation, low-reflectance optical coatings, and even protection of aluminum against reactive plasmas in semiconductor manufacturing equipment. In summary, PEO is a very versatile technology which presents a wide range of solutions to surface engineering challenges for light metals. Dr. Curran is with the Department of Materials Science & Metallurgy, Cambridge University, Pembroke Street, Cambridge, and is the global research and development manager for Keronite International Ltd. He can be reached at jac64@cam.ac.uk Comments are reviewed by moderators before they appear to ensure they meet Products Finishing’s submission guidelines.

Detection of anodising layer on aluminium - Metal and Metallurgy engineering - Eng-Tips Detection of anodising layer on aluminium 5 Detection of anodising layer on aluminium Detection of anodising layer on aluminium (OP) 27 Jan 03 11:01 I have a little coated plate (50x50x0.6mm). After removing the paint, I have been asked to proove that there also is a anodising layer. I was thinking about dipping in nitric acid and watching how much material seem to come off. But I only have a small plate. If this does not work, I don't have a second chance. Anyone? RE: Detection of anodising layer on aluminium 27 Jan 03 11:45 I see two ways to do this: 1) make a metallurgical cross-section, then polish and etch it. An anodized layer will be detectable. 2) measure the indentation hardness (either surface or cross-section). The anodized layer will be harder than the base metal. You will need to choose a proper indenter and force based upon the materials and geometry.

RE: Detection of anodising layer on aluminium 27 Jan 03 20:10 Two easy methods: 1) Check for electrical continuity -- anodize is a good insulator, whereas a probe will easily break through the oxide film on bare aluminum. 2) Try scratching with a piece of steel -- aluminum will scratch, anodize won't. RE: Detection of anodising layer on aluminium davefitz (Mechanical) 28 Jan 03 10:00 Anodized aluminum has an infrared emmissivity index of greater than 0.85, while non anaodized aluminum has an infrared emmissivty less than 0.15. Try using a infrared radiation meter, adjust the settings to read emmissivity, and voila! RE: Detection of anodising layer on aluminium TEV (Materials) 28 Jan 03 16:17 Scanning electron microscopy with energy dispersive x-ray spectroscopy will tell you if you have an anodized layer. You may need comparison samples known to be anodized and not anodized.

RE: Detection of anodising layer on aluminium 28 Jan 03 16:55 TEV, For SEM to detect anodic coating, you don't need an EDS analyzer or standards. The anodic coating, being a nonconductor, will immediately charge up and deflect the electron beam all over the place. When studying anodic coatings via SEM, it is necessary to first prepare the sample by evaporating a conductive film of carbon (or gold). gvh, For a good answer by non-destructive means, find a local anodizing company and ask them to measure the anodize thickness. It is quick & easy using an eddy current tester, and if you know the alloy, it can be accurate to 0.00001 inch. RE: Detection of anodising layer on aluminium AlanD (Materials) 29 Jan 03 00:03 Using a cross sectional view of the sample, the anodized layer should be clearly visible under a microscope after a HF etch or clearly visible in an SEM, no coating required.

RE: Detection of anodising layer on aluminium TEV (Materials) 29 Jan 03 17:37 Kenvlach - electron beam charging depends on anodization (oxide) thickness, primary beam voltage, and imaging mode. It is actually possible to image areas of differing oxide thickness in an SEM by varying the accelerating voltage. As you decrease the voltage, the secondary electron emission coefficient of oxides increases much more rapidly than that of the metal, and beam penetration also descreases. So, as you lower the accelerating voltage, thinner and thinner oxide areas become brighter in the secondary electron imaging mode. I have used this to plot oxide distribution on aluminum. RE: Detection of anodising layer on aluminium 29 Jan 03 18:25 TEV, I think my comment is generally true, to study a non-conductive surface by SEM requires a conductive coating. If you studied the initial stages of oxide formation on aluminum, no C-coating would be needed since the aluminum would suffice. And of course for edge view as mentioned by Alan, you have adjoining conductive layer.

Re SE image, I recall that rastering the beam over a large area also helps reduce charging, but I had to use sufficient voltage (10 KeV?) to do Al, Zn & Mg mapping of the surface oxide for the alloy in question. I would like to see your reference for studying anodize w/o coating. But, we're getting sidetracked. gvh has this small plate & he wants an NDT for determining anodize presence. He clearly doesn't want cross-sectioning or need a method that costs more than replacing his plate. Other than the IR method mentioned by davefitz, I feel my suggestions are the only practical ones. gvh, please print out this thread, take it with your plate to an anodizing shop and beg a free thickness test (better than bothering with billing) and ask their opinion of the various proposed methods. RE: Detection of anodising layer on aluminium 30 Jan 03 03:43 Thanks you all. I have tried to measure electrical resistance, but no resistance could be measured with a normal multimeter. So if there is an anodised layer, it will be very small.

Since the component has very high corrosion resistance in Salt spary test (this was done on an other part from the the same plate) we know there must be some layer. But too small for us to detect, even by light microscopy. I do have an old SEM (no edx)so my next step will be to try this, with a very low accelerating voltage. I will keep you all informed here. Extra remark for this site: I really did not think at first that so many people would respond. Now I have tried it, am very ethousisatic about this site. Keep up the good work! RE: Detection of anodising layer on aluminium 30 Jan 03 11:27 kenvlach writes: gvh has this small plate & he wants an NDT for determining anodize presence. He clearly doesn't want cross-sectioning or need a method that costs more than replacing his plate. Other than the IR method mentioned by davefitz, I feel my suggestions are the only practical ones. To which I have the following comments: 1. gvh made no such claim that only NDT methods could be used.

Obviously he/she does not want to destroy the entire sample, but that does not rule out some destructive testing. 2. The second sentence is complete conjecture. It is quite common to perform analyses on a component(s) for a variety of reasons (premature failure, quality control, customer demand, etc.) that cost more than the price of replacing an individual component. 3. Placing value judgements on the quality of the other replies based on one's limited perspective is borderline offensive. All of the suggested methods have merit, and it is up to gvh to make a decision on which method to pursue, if any. RE: Detection of anodising layer on aluminium TEV, OK, you've made your point. I admit, I don't know what's inside gvh's head. I interpreted his concern about damaging the plate as a cost issue, whereas it could be a forensic examination. Your remark "one's limited perspective is borderline offensive" is rather offensive, as I have done plenty of slice & dice, mounting & polishing, SEM & EPMA work, so I have used all the methods mentioned except the one by IR. I was speaking from years of experience in the aluminum anodizing business regarding what was practical.

The vast majority of customers that I have dealt with were concerned with cost. And compared to the SEM/TEM/EPMA methods my research group used to study oxide films, alumina-aluminum alloy composites, etc., plus those I later used to study anodic coatings, I can definitely say that there are faster, easier and less expensive means to test for anodize presence and thickness. We routinely use $2 pen-size continuity testers having a battery & light to verify that racks are free of anodize, or whether a casting has been completely descaled, etc, & sometimes a VOM to check whether a scratch has penetrated the anodize layer on a part. Eddy current testers are used for QC, i.e., for fast, accurate thickness measurements. RE: Detection of anodising layer on aluminium 30 Jan 03 14:07 I apologize, the above post was in response to TVP' post. And, I suggest that as gvh didn't find any electrical resistance, all of this is moot.

RE: Detection of anodising layer on aluminium 30 Jan 03 14:25 kenvlach, I believe this thread is not moot and that some of your comments are speculative, not factual. gvh never stated how the paint was removed - if it were sand blasted, maybe some aluminium oxide coating was removed. gvh didn't state details regarding the multimeter measurements - if the electrodes were sharp-pointed, perhaps they could deform/fracture a thin oxide layer. If "proof" of a coating is necessary, then a combination of the suggested techniques may be necessary. I don't think you should be offended by TVP's comments - take them as constructive advice since we all have limited perspectives that influence our discourse. RE: Detection of anodising layer on aluminium Advancing the surface technology of materials | University of Cambridge Advancing the surface technology of materials Research News Advancing the surface technology of materials Research at the Gordon Laboratory is opening up an important new area for the surface engineering of materials. With the benefit of research such as this, PEO coatings of the future will afford greater degrees of durability in demanding environments.

For nearly a century, materials manufacturers have known that metals exposed to demanding environments can be protected by a simple yet extremely effective process known as anodising. By immersing metals such as aluminium, titanium, zinc and magnesium in a solution that conducts electricity and applying an electric current, the natural oxide on the metal surface thickens and creates a protective outer layer. Although anodised layers are typically only a few microns in depth and are porous, this simple process can alter the surface appearance of a metal and, importantly, vastly improve its resistance to corrosion and wear. Today, anodised metals enjoy varied and far-reaching applications stretching from construction, through the automotive industry, to consumer products. More recent modifications have improved this process still further: by using stronger electric fields in a process known as plasma electrolytic oxidation (PEO), thermal and electrical protection can now be added to the list of attributes. Coatings produced in this way on aluminium, for instance, incorporate a very hard, crystalline phase of aluminium oxide called corundum – known in its naturally occurring form as the minerals sapphire and ruby – that is second only in hardness to diamond.

Lightning strikes As the electric field is increased to achieve these harder, thicker and more durable coatings, localised electric discharges or ‘sparking’ occurs – perhaps best likened to micro-lightning strikes within the forming coat. In the past decade, attention has focused on what exactly is happening during sparking. How do these phenomena dictate the microstructure of the coating material? Is it possible to control them and even to use them to influence the properties of the oxide coat? Questions such as these are being investigated by Professor Bill Clyne in the Gordon Laboratory, Department of Materials Science and Metallurgy, in close collaboration with Dr James Curran seconded from Keronite International Ltd ( ), a Cambridge-based industrial manufacturer that is pioneering PEO technology. Taming the spark The phenomenon of sparking has historically been regarded as destructive, imposing a limit on the growth rate and the thickness of anodised layers, and generating defects and microcracks in the material. But, as research in the Gordon Laboratory has progressed, it’s become clear that the discharges can in fact be ‘tamed’ and used beneficially.

So long as a suitable electrolyte and electrical control circuitry is employed, the discharges can be made to occur repeatedly in a controlled fashion over the surface of the component, and the conditions of high local pressure and temperature within the resulting plasma can be used to modify the structure of the growing oxide layer and enhance its properties. Controlled coatings Rather than being unwanted defects, the ultra-fine, interconnected porosity and networks of microcracks within PEO coatings appear to play an unsung role. The researchers have found that this porosity is essential for the process to continue. In fact, the so-called defects actually confer important beneficial effects on the coatings, improving their thermal insulation, enhancing their strain tolerance (giving better adhesion) and leading to excellent ‘keying’ with overlays such as paints and sealants. The focus now has turned towards analysing the electrical and spectroscopic characterisation of discharges at the level of single events, with the aim of developing a process model that has the capability of predicting and controlling the microstructural features that determine coating properties. With the benefit of research such as this, PEO coatings of the future will afford greater degrees of durability in demanding environments.

For more information, please contact the authors Professor Bill Clyne ( twc10@cam.ac.uk ) or Dr James Curran ( jac64@cam.ac.uk ) at the Gordon Laboratory, Department of Materials Science and Metallurgy. This work is licensed under a Creative Commons Licence . If you use this content on your site please link back to this page. Published Top five questions about anodizing architectural tube - The Fabricator Top five questions about anodizing architectural tube Top five questions about anodizing architectural tube Science and nature come together By: Tammy Schroeder Anodizing combines with nature to create one of the world's best metal finishes. It is the process of electrochemically controlling, accelerating, and enhancing oxidation of an aluminum tube, creating a durable, scratch-resistant coating on the surface comparable to a sapphire. Architectural anodize finishes are limited to certain colors, and it is impossible to produce a perfect color match. Perform as much bending and forming as possible before finishing. During anodizing, the aluminum oxide layer is made thicker by passing a direct current through a sulfuric acid solution, with the tube serving as the anode, the negative electrode.

The current releases hydrogen at the cathode, the positive electrode, and oxygen at the surface of the aluminum anode, creating a buildup of aluminum oxide. Because of its strength and durability, anodized aluminum is used in a number of applications. Many structures and buildings have anodized aluminum in places where the metal framework is exposed to the elements, such as handrails, metal coping, column covers, HVAC ducts, and decorative metalwork. 1. What Is Anodizing? Anodizing combines with nature to create one of the world's best metal finishes. It is the process of electrochemically controlling, accelerating, and enhancing oxidation of an aluminum tube, creating a durable, scratch-resistant coating on the surface (see lead image). Preparation. The anodizing process typically begins with the aluminum tube being cleaned in a nonetching alkaline chemical cleaner to remove all shop dirt, water, and soluble oils that may have accumulated on the tube's surface during handling and manufacturing. After it is cleaned, the tube is ready for caustic etching. The caustic etch process produces a matte finish and also minimizes minor surface imperfections, such as light die lines and travel marks.

Caustic etching does not eliminate all surface imperfections. A good rule is if the imperfection can be felt with a fingernail before the tube is anodized, it is likely that it will not be removed by caustic etching. The tube is then desmutted and rinsed to remove residues left from the caustic etch. This is the final preparation stage prior to anodizing. The Process. The sulfuric acid (Type II) anodizing process produces a protective and decorative oxide finish on aluminum tube. The aluminum oxide layer is made thicker by passing a direct current through a sulfuric acid solution, with the tube serving as the anode, the negative electrode. The current releases hydrogen at the cathode, the positive electrode, and oxygen at the surface of the aluminum anode, creating a buildup of aluminum oxide. Coloring. If the tube requires coloring, it is moved to a two-step electrolytic coloring tank. Tin metal is electrochemically introduced into the anodic pores to produce bronze tones ranging from light champagne to black (see Figure 1). After it is anodized and colored, the material is sealed in a midtemperature hydrothermal seal and is then given a final hot water rinse.

This final, important step ensures that the high-quality anodized finish maintains its color for many years. 2. What Are the Advantages and Disadvantages of an Anodize Finish? Advantages. An anodize finish satisfies the requirements for aluminum tube: Durability. Anodize offers a hardness and scratch resistance comparable to a sapphire, the second-hardest substance after a diamond. The aluminum oxide that is formed is an integral part of the substrate and is much harder than the aluminum it replaces. This is why it gives excellent wear and abrasion resistance. Because the coating is an integral part of the substrate, the anodic coating will not chip, peel, or flake over time. Color stability. Exterior anodic coatings provide good stability to ultraviolet rays and do not chip or peel. The process is repeatable. Ease of maintenance. Scratches and wear from fabrication, handling, installation, surface cleaning, and usage are rare. Rinsing or mild soap and water cleaning usually will restore an anodized surface to its original appearance. Mild abrasive cleaners can be used to remove more difficult deposits. Anodized surfaces do not show fingerprints. Appearance. Anodizing offers a large and increasing number of gloss and color choices.

Unlike most other finishes, anodizing allows the aluminum to maintain its metallic appearance (see Figure 1). Cost. The initial finishing cost is relatively minimal; so too are its maintenance costs. Health and safety. Anodizing is a safe process that is not health-hazardous: An anodized finish is chemically stable, will not decompose; is nontoxic; and is heat-resistant to the melting point of aluminum (1,221 degrees F). Because the anodizing process replicates naturally occurring oxidation, it is nonhazardous and produces no harmful or dangerous byproducts. Disadvantages. A few challenges associated with an anodize finish are: Color choices. Architectural anodize finishes are limited to certain colors, including clear (silver), black, champagne, and traditional bronze tones. Color match. The color obtained in the anodize process depends on many factors, such as alloy, temper, and shape; therefore, it is impossible to produce a perfect color match. Touchups. Anodize finishes are factory-applied, so field repair touchups must be done with paint. A paint finish will never match an anodize finish well. Visible surface imperfections.

Anodize is an integral part of the aluminum; therefore, heavy die lines, dents, and dings on the aluminum part will show through the finish. 3. Which Classifications Apply to Anodizing? Architectural Class I and Class II anodic coatings are designations created by the American Architectural Manufacturers Association (AAMA) for the purpose of codifying the specification of anodized aluminum (see Figure 2). Figure 1. Tin metal is electrochemically introduced into the anodic pores to produce bronze tones ranging from light champagne to black. Class I coating has a mil thickness of 0.7 (18 microns) or greater. Class II coating has a minimum mil thickness of 0.4 (10 microns). Figure 2 Low Very Low A Class I coating is a high-performance anodic finish used primarily for exterior building products and other products that must withstand continuous outdoor exposure. A Class II coating is a commercial anodic finish recommended for interior applications or light exterior applications receiving regularly scheduled cleaning and maintenance, such as storefronts. The AAMA 611 specification for anodized architectural aluminum is the most commonly used, although other specifications may be followed as required. 4.

What Material Precautions Should Be Taken? Preparing the aluminum tube correctly for anodizing will reduce waste and produce the optimal finish: Maintain material consistency. The easiest way to ensure consistency in aluminum tube is to work with one metal source/extruder per project. Do not mix aluminum alloys. Mixed alloys, or even mixed tempers, will not produce uniform results. For best results, use 6063 alloys for extrusions and 5005 for sheet stock. Perform as much bending and forming as possible before finishing. Anodic films are very hard. As a result, most postproduction bending causes the film to "craze," which produces a series of small cracks in the finish, giving it a spider web appearance. Store material safely. Prior to an-odizing, store the mill aluminum tubes in a dry and controlled environment. It is important to do this throughout the fabricating and finishing processes. Do not allow moisture to build up between the tubes, because this will cause severe corrosion, known as white rust, which cannot be removed in the finishing process. Avoid using tape or adhesives. Tape or adhesive on the aluminum may leave a residue that cannot be removed in the anodize process.

Discuss the desired specification with the finisher. In the architectural industry, the most recognized specification is AAMA 611-98. If specific parameters are required, it is important that the finisher be furnished with the desired requirements to ensure that the finish satisfies expectations. Be aware of anodizing's effect on welds. The weld on a tubular component has a different color than the unjoined areas of the component. The heat generated during the welding process can disturb the metallurgy on nearby metal, or heat-affected zone (HAZ), and cause a localized discoloration after anodizing. Use the proper 5356 alloy welding wire and the lowest heat possible. Use proper drainage. Holes are essential to prevent solution from being trapped. Drainage of the solution allows trapped gas to escape from the tubes. Even the tightest of welded joints will allow anodized chemicals to weep out. Let the finisher know where parts can be racked. There are a variety of ways anodizers rack tubular components, from welding material to spline bars to a screw-down bolt system. In any case, contact marks are visible on the aluminum.

It is important to define what is acceptable and what is unacceptable regarding exposed surfaces and rack marks. Save on material handling by cutting and punching aluminum after it's finished. Cutting and punching may be performed either before or after the material has been anodized. Cutting and punching cut-to-size tube after it is sent to the anodizer may be more expensive because of the extra material handling and load size requirements. Cutting and punching before anodizing may be more cost-effective, and doing so means that the cut and punched edges will be anodized also. Use good shipping practices. The fabricator should package the tubes carefully before shipment to the finisher to ensure the metal arrives dry and free of scratches and dents. Avoid sending the finisher aluminum tubing with scratches, dings, heavy die lines, and die pickup. These metal quality defects will show through the anodize finish; tubes without defects will exhibit a higher-quality finish. 5. How Should Anodized Aluminum Tubing Be Cared for After Fabrication? Initiate cleaning procedures for aluminum as soon as it is practical to do so after installation is completed to remove construction soils and accumulated environmental soils and discolorations.

The simplest way to remove light soils is to flush the surface with water using moderate pressure. After the surface is air-dried, if soil is still present, scrub it with a brush or sponge and try concurrent spraying with water. If soils still adhere, use a mild detergent cleaner with brushing or sponging. Use uniform pressure to wash it—first horizontally then vertically. Thoroughly rinse the surface with clean water. You must take certain precautions when cleaning anodized aluminum surfaces. First, identify the finish to select the appropriate cleaning method. Never use aggressive alkaline or acid cleaners. Avoid cleaning hot, sun-heated surfaces because possible chemical reactions will be highly accelerated, resulting in nonuniform cleaning. Strong organic solvents, while not harmful to anodized aluminum, may extract stain-producing chemicals from sealants and affect the function of the sealants. Strong cleaners should not be used on window glass and other components on which it is possible for the cleaner to come in contact with the aluminum. Avoid excessive abrasive rubbing because it could damage the finish. For added protection, apply wipe-on surface protectants, which are estimated to provide protection for 12 to 24 months in the harshest environments and make subsequent maintenance easier.

One of the best steps a tube fabricator can take to ensure a quality tube finish is to form a good working relationship with the finisher. Ongoing, open communication will be among the most valuable factors in minimizing waste and maximizing results. Tammy Schroeder Powder Manufacture | European Powder Metallurgy Association (EPMA) and the atomization of molten metal Of these the last three account for the bulk of the powders used. Solid State Reduction This has been for long the most widely used method for the production of iron powder. Selected ore is crushed, mixed with carbon, and passed through a continuous furnace where reaction takes place leaving a cake of sponge iron which is then further treated by crushing, separation of non-metallic material, and sieving to produce powder. Since no refining operation is involved, the purity of the powder is dependent on that of the raw materials.The irregular sponge-like particles are soft, and readily compressible, and give compacts of good green strength. Refractory metals are normally made by hydrogen reduction of oxides, and the same process can be used for copper.

Electrolysis By choosing suitable conditions - composition and strength of the electrolyte, temperature, current density, etc., many metals can be deposited in a spongy or powdery state. Extensive further processing - washing, drying, reducing, annealing and crushing may be required. Copper is the main metal to be produced in this way but chromium and manganese powders are also produced, by electrolysis. In these cases, however, a dense and normally brittle deposit is formed and requires to be crushed to powder. Electrolytic iron was at one time produced on a substantial scale but it has been largely superseded by powders made by less costly processes. Very high purity and high density are two distinguishing features . Atomization In this process molten metal is broken up into small droplets and rapidly frozen before the drops come into contact with each other or with a solid surface. The principal method is to disintegrate a thin stream of molten metal by subjecting it to the impact of high energy jets of gas or liquid. Air, nitrogen and argon are commonly used gases, and water is the liquid most widely used.

Gas atomization A commercial scale plant was set up in Japan to produce iron powder using paraffin as the atomizing liquid the object being to keep the surface oxygen content as low a possible. The process was technically successful, but the advantages did not justify, in commercial terms, the extra cost involved. However, interest has not entirely evaporated and work is going on elsewhere. Shapes of gas atomization powders By varying the several parameters: design and configurations of the jets, pressure and volume of the atomizing fluid, thickness of the stream of metal etc. - it is possible to control the particle size distribution over a wide range. The particle shape is determined largely by the rate of solidification and varies from spherical, if a low heat capacity gas is employed, to highly irregular if water is used. In principle the technique is applicable to all metals that can be melted, and is commercially used for the production of iron, copper, including tool steels, alloy steels, brass, bronze and the low-melting-point metals, such as aluminium, tin, lead, zinc, cadmium. The readily oxidisable metals, for example chromium-bearing alloys, are being atomized on an increasing scale by means of inert gas, specially argon.

In addition, there are several other processes that are finding increasing application, an important one being centrifugal atomization in which droplets of molten metal are discharged from a rotating source. There are basically two types of centrifugal atomization processes: in one a cup of molten metal is rotated on a vertical axis at a speed sufficient to throw off droplets of molten metal, or a stream of metal is allowed to fall on a rotating disc or cone; in the other a bar of the metal is rotated at high speed and the free end is progressively melted e.g. by an electron beam or plasma arc. This latter process is called the Rotating Electrode Process (REP), and the bar may be rotated either on a horizontal or on a vertical axis. A special advantage of these processes is that they can be carried out in a sealed vessel in a controlled atmosphere - even vacuum - and thus produce 'clean' powders of highly reactive metals. With the REP process the avoidance of contact with refractory is a potent means of reducing the number of non-metallic inclusions in the powder, and in components manufactured from the powder.

Atomization is particularly useful for the production of alloys in powder form, since the constituents metals are fully alloyed in the molten state. Thus each powder particle has the same chemical composition. Additionally the process is used to produce compositions such as copper-lead, in which the lead, though soluble in the liquid state, comes out of solution on solidification. If a casting of such an alloy is made, serious segregation of the lead results, but if the liquid is atomized, the end product is copper powder containing a very fine and uniform distribution of lead inclusions within each powder particle. Mechanical Comminution Brittle materials such as inter-metallic compounds, ferro-alloys - ferro-chromium, ferro-silicon, etc. are pulverised mechanically in ball mills, and a process known as the Coldstream Process is finding increasing application for the production of very fine powders such as are required for injection moulding (qv).

In this process, granular material, which may be coarsely atomized powder, is fed in a stream of gas under pressure through a venturi and is cooled and thereby embrittled by the adiabatic expansion of the gas before impinging on a target on which the granules shatters Chemical Processes Thermal decomposition of a chemical compound is used in some cases, a notable one being nickel carbonyl. This Carbonyl Process was originally developed as a means of refining nickel, crude metal being caused selectively to react with carbon monoxide under pressure to form the carbonyl which is gaseous at the reaction temperature and which decomposes on raising the temperature and lowering the pressure. The same process is used for iron, and carbonyl iron powder finds small scale application where its very high purity is useful. Recently, demand for very fine powders for the injection moulding process has given a considerable impetus to the carbonyl process. Typically the particle size of carbonyl iron powder is 1 - 5 m, but, as in the case of nickel, it can be tailored to suit particular requirements. Another case of thermal decomposition is platinum powder of which is made from sponge produced by heating salt - platinum ammonium chloride.

In the Sherritt-Gordon process, nickel powder is made by hydrogen reduction of a solution of a nickel salt under pressure. Chemical precipitation of metal from a solution of a soluble salt is used in other cases - e.g. silver, powder of which is produced by adding a reducing agent to a solution of silver nitrate. This is, of course, the same basic process as is used to produce black and white photographs. Powder Characteristics The further processing and the final results achieved in the sintered part are influenced by the characteristics of the powder: particle size, structure and surface condition. A very important parameter is the apparent density (AD) of the powder, i.e. the mass of a given volume, since this strongly influences the strength of the compact obtained on pressing. The AD is a function of particle shape and the degree of porosity of the particles. The choice of powder characteristics are normally based on compromise, since many of the factors are in direct opposition to each other: An increase in the irregularity and porous texture of the powder grain, i.e. decrease in apparent density, increases the reduction in volume that occurs on pressing and thus the degree of cold-welding, which, in turn, gives greater green strength to the compact.

This increase in contacting surfaces also leads to more efficient sintering. The same process is used for iron, and carbonyl iron powder finds small scale application where its very high purity is useful. Recently demand for very fine powders for the injection moulding process has given a considerable impetus to the carbonyl process. Typically the particle size of carbonyl iron powder is 1 - 5 m, but, as in the case of nickel, it can be tailored to suit particular requirements. Additionally the greater reduction in volume necessary to give the required green density may require greater pressure and consequently larger presses and stronger dies. The ease and efficiency of packing the powder in the die depends to a large extent on a wide particle size distribution. So that the voids created between large particles can be progressively filled with those of smaller size. Fine particle sizes tend to leave smaller pores which are easily closed during sintering. An excess of fines, however, reduces flow properties with the results already detailed above. The purity of the powder is critically important . Impurity levels which can be tolerated depend to a large extent on the nature and state of combination of the substances concerned.

For example, the presence of combined carbon in iron tends to harden the matrix so that increased pressures are required during compaction. Free carbon, however, is often an advantage, acting as a lubricant during the pressing operation. Most metal powder grains are coated by a thin oxide film, but in general these do not interfere with the process, since they are ruptured during the pressing operation to provide clean and active metal surfaces which are easily cold-welded. Their final reduction under the controlled sintering atmosphere is essential for complete metal bonding and maximum strength. Stable oxide films or included oxide particles, such as SiO2 and Al2O3 are more serious, since these are generally abrasive and lead to increased tool wear. Furthermore they cannot be reduced during subsequent sintering and their presence may adversely affect the mechanical properties especially impact strength of the finished part.

Question: In metallurgy, the coating process known as anodising is associated with which metal?
Answer: ALUMINIUM